Optical fibers are of interest for optical communications systems as well as for other applications such as interferometric sensors. Optical fibers typically comprise a high refractive index core which is surrounded by a low refractive index cladding. The essential components of an optical communications system as presently contemplated are a light source and a photodetector optically coupled to each other by means of the optical fiber. The highest capacity optical communications systems will be obtained with use of single mode optical fibers. It is readily understood by those skilled in the art that although optical fibers are referred to as single mode fibers, they are in reality capable of supporting two modes having orthogonal polarizations.
Other types of optical devices in addition to those just mentioned also promise to be useful in optical communications systems. For example, polarization converters are optical devices which change the state of polarization of a propagating electromagnetic wave. Such devices may be used in optical communications systems as optical modulators or as wavelength selective components if the polarization conversion efficiency is wavelength dependent. Because of the versatility and usefulness of such devices, they promise to be of great importance in single mode optical communications systems. As such, much effort has been devoted to developing efficient polarization converters and wavelength selective components.
Wavelength selective filters have been known for some time. For example, a Lyot filter comprises a plurality of birefringent wave plates. Each plate is positioned between parallel linear polarizers and has twice the retardation of the previous polarizer. Another type of filter is a Solc filter which has a plurality of half wave plates with the fast axis of each plate rotated about the optic axis by a specified angle which varies from plate to plate in a specified manner. Embodiments of Lyot and Solc filters using bulk optics are typically unwieldy, i.e., require a lot of space, and are not expediently fabricated for communications systems.
Another approach to developing a polarization converter, which is also a wavelength selective filter, uses integrated optics. One integrated optics approach is described in Applied Physics Letters, 36, pp. 513-515, Apr. 1, 1980. This article describes an efficient planar waveguide electro-optic TE-TM mode converter and wavelength filter. In the particular embodiment of the mode converter described in detail, wavelength selective polarization conversion is achieved using a strip Ti diffused lithium niobate waveguide. Efficient coupling between the transverse electric and magnetic modes was obtained by using periodic finger electrodes to yield the desired phase matching between the TE and TM modes. The strongly wavelength dependent nature of the polarization conversion resulting from the highly birefringent nature of lithium niobate makes the device useful in applications such as multiplexing and demultiplexing.
Although the integrated optics approach to polarization converters promises to be useful for many applications, there are other applications in which it would be desirable to make an optical fiber a part of the polarization converter. Such converters can be termed in-line rotators or converters. The fibers used in this type of converter will necessarily be single mode fibers. In practice, many optical fibers, either deliberately or through variations in manufacturing processes, exhibit optical birefringence as one of the principal fiber axes has an effective index of refraction different from that of the other principal axis. Thus, in-line optical fiber polarization converters are conceptually feasible. In such converters, polarization conversion occurs by a type of phase-matched coupling similar to that in the planar waveguide electro-optic mode converter previously discussed.
One single mode fiber optical polarization rotator is described in Applied Optics, 18, pp. 1857-1861, June 1, 1979. The approach described in this article uses a birefringent single mode optical fiber and mechanically twists the fiber in alternating directions on successive half wave fiber sections, i.e., it uses externally induced stresses. The authors believed that the operative mechanism resulting in polarization conversion was not only the fiber birefringence but also optical activity induced by the mechanical twists. See, also, U.S. Pat. No. 4,341,442 issued on July 27, 1982 which describes an optical filter using a twisted birefringent optical fiber.
Another approach uses Faraday rotation in birefringent optical fibers and is described in Applied Optics, 19, pp. 842-845, Mar. 15, 1980, by R. H. Stolen and E. H. Turner. This paper demonstrated that Faraday rotation in optical fibers can be obtained by using, for example, alternating regions of magnetic field. Other techniques, such as periodically spaced magnetic field regions, were also described.
Another single mode fiber filter is described in Optics Letters, 5, pp. 142-144, April 1980. Construction of a filter by cutting and splicing birefringent fiber lengths to achieve the desired retardation of one polarization with respect to the other is disclosed. A tunable filter embodiment is also suggested using externally induced stresses such as those already described. The authors believed that the twist produced optical activity or circular birefringence.
Another optical fiber filter which is based upon refractive index variations in the fiber is described in Applied Physics Letters, 32, pp. 647-649, May 15, 1978. A tunable filter was formed within the optical fiber by light induced refractive index changes in a silica core doped with germania.
The in-line fiber polarization rotators are often preferred at the present time to the integrated optic polarization rotators because they have less loss as they are fabricated from less lossy compositions than are the integrated optic devices and there are fewer reflections at the interfaces. Perhaps even more significantly, in-line rotators can theoretically be fabricated in very compact embodiments. However, the in-line fiber polarization rotators described are generally not totally satisfactory for all applications because external means are required to produce desired polarization conversion and these means are difficult to fabricate in a uniform manner or the fiber must be cut into sections which are then joined together after rotation.